In the use of a priority hydraulic circuit having a lockout valve and a control valve, it is desirable to prevent priority fluid flow from a priority valve to an associated work system in response to demand of the control valve where said demand is created as a result of a primary work system being locked out of or blocked from operation by the lockout valve. In this manner, the priority valve creates priority fluid flow to a secondary work system and prevents a pressurized fluid source from pumping against relief pressure of the circuit owing to the locked out condition of the primary work system.
U.S. Pat. No. 3,642,019 which issued to Kramer on Feb. 15, 1972, shows an embodiment of a hydraulic circuit having a priority valve controllably passing fluid from primary and secondary outlets. A control valve causes the priority valve to direct all fluid through the secondary outlet in response to pressure in the primary outlet reaching a preselected magnitude.
U.S. Pat. No. 4,074,528 which issued to Lourigan on Feb. 21, 1978, discloses a pilot operated priority valve which passes fluid from a primary outlet to a steering control valve. The steering control valve passes a fluid signal to the priority valve in order to provide flow to said steering control valve in response to the demand of said valve.
U.S. Pat. No. 3,386,344 which issued to Junck on June 4, 1968, shows another embodiment of a priority valve which passes fluid to a steering control valve. Priority to the control valve is assured by fluidly communicating the line passing fluid from the priority valve to the control valve with one side of a valve spool of the priority valve.
U.S. Pat. No. 3,550,505 which issued to Byers, Jr. on Dec. 29, 1970, shows yet another embodiment of a priority valve passing fluid to a control valve. A fluid line from the control valve to a spring chamber of the priority valve assures priority of fluid flow to the control valve in response to demands of the control valve.
U.S. Pat. No. 3,953,040 which issued to Unruh on Apr. 27, 1976, shows a leveling control valve connecting the axle and frame of a work vehicle for automatically maintaining the axle at a preselected position relative to frame by automatically passing fluid to suspension cylinders in response to relative movement of the axle and frame.
U.S. Pat. No. 3,865,135 which issued to McWilliams on Feb. 11, 1975, shows the use of manually operated lockout valve which is used to block fluid flow to suspension cylinders of a work vehicle in order to lock out the operation of the vehicle suspension.
For example, a cushioning or suspension system is generally provided on a tractor portion of a scraper in order to improve the controllability and ride of the scraper. The suspension system is commonly based upon hydraulic cylinders connected to the frame and axle of the scraper for controlling relative movement of said frame and axle. A priority valve is used to establish the priority of fluid flow from a fluid source to the system relative to other work systems of the scraper. A control valve is used to automatically, controllably respond to the demands of the suspension system and provide the necessary fluid flow to or from said system. The control valve also delivers a signal to the priority valve in order to assure the priority of fluid flow to the suspension system.
In such suspension systems, a lockout valve is preferably added to the hydraulic circuit to provide for selective operation of the suspension system. For example, it is desirable that the suspension system be locked out of or blocked from operation during loading cycles of the scraper. This results in better control and handling during the loading operation. However, in locking out the suspension system, said system can be maintained at a position at which the control valve continues to signal the priority valve to maintain priority fluid flow to the inoperative suspension elements. This can result in the pressurized fluid source pumping fluid against the relief pressure of the hydraulic circuit creating energy loss, heating of the oil, and lack of fluid flow from the priority valve to secondary work systems.
Therefore, it is desirable to automatically controllably block the signal from being delivered by the control valve to the priority valve in response to the suspension system being locked out of operation for establishing priority to secondary systems and preventing fluid flow to the inoperative suspension system.